1. Field of the Invention
This invention relates to an organic electroluminescent device having excellent light emitting properties.
2. Description of the Related Art
An organic electroluminescent device (which will hereinafter be called xe2x80x9corganic EL devicexe2x80x9d) is a light emitting device which makes use of the principle that when an electric field is applied, a fluorescent material emits light in response to the charge recombination of holes injected from an anode and electrons injected from a cathode. Since C. W. Tang et al. of Eastman Kodak Company made a report on a low-voltage-driven organic EL device using a double layered structure (C. W. Tang, S. A. Vanslyke, Applied Physics Letters, 51, 913(1987) and the like), studies on an organic EL device and materials have been briskly carried out.
Tang et al. applied tris(8-hydroxyquinolinol)-aluminum to a light emitting layer and a triphenylamine derivative to a hole transport layer. The laminate structure is accompanied with such advantages as an improvement in the injection efficiency of holes into a light emitting layer; blocking of electrons injected from a cathode, which increases the efficiency of exciton production from charge recombination; and confine of the excitons into the light emitting layer. As an organic EL device structure, a double layered device composed of a hole injecting and transport layer and an electron transporting and light emitting layer or a triple layered device composed of a hole injecting and transporting layer, a light emitting layer and an electron injecting and transporting layer are well known. In order to increase the recombination efficiency of injected holes and electrons, various improvements in the device structure or fabrication process have been introduced to such a multi-layered device.
As the hole transport material, triphenylamine derivatives such as 4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino) triphenylamine known as a starburst molecule and aromatic diamine derivatives such as N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diamine are well known (ex. Japanese Patent Application Laid-Open Nos. 20771/1996, 40995/1996, 40997/1996, 543397/1996 and 87122/1996).
As the electron transporting material, oxadiazole derivatives and triazole derivatives and the like are well known.
As the light emitting material, known are chelate compounds such as tris(8-quinolinolate)aluminum complex, coumarin derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives, oxadiazole derivatives and the like. Since the various color luminescent materials from blue to red region are reported, there increased expectation for industrialization of a full color organic EL display (ex. Japanese Patent Application Laid-Open Nos. 239655/1996, 138561/1995, 200289/1991 and the like).
In recent days, an organic EL device with high brightness and long life has been disclosed or reported, but it is not always sufficient for manufacturing actual devices yet. Accordingly, there is a strong demand for the development of materials exhibiting high performance.
An object of the present invention is to provide an organic EL device including a material represented by the formula (I), which potentially has high brightness and long life.
The present inventors have carried out an extensive investigation with a view to dissolving the above-described problem. As a result, it has been found that compared with the conventional device, an organic EL device fabricated using a compound represented by the formula (I) as a light emitting material performs higher brightness and has a longer life when continuously driven. It has also been found that the above-described material has high carrier transporting properties and compared with the conventional device, an organic EL device fabricated using the material as a hole transport material or electron transport material or an organic EL device fabricated using a mixed thin-film of the material and another hole or electron transport material performs higher brightness and has a longer life when continuously driven, leading to the completion of the invention.
In the present invention, there are thus provided an organic electroluminescent device comprising an anode, a cathode and, between said anode and cathode, one or more than one organic thin-film layers including a light emitting layer, wherein at least one of said layers contains, either singly or as a mixture, a material represented by the following formula (I): 
wherein R1 to R4 each independently represents a hydrogen atom, a halogen atom, an amino group, a hydroxyl group, a cyano group, a nitro group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aromatic hydrocarbon group, a substituted or unsubstituted aromatic heterocyclic group, a substituted or unsubstituted aralkyl group, or a substituted or unsubstituted aryloxy group; or R1 to R4 may be coupled together to form a ring; and the above-described organic electroluminescent device having a hole transport layer or electron transport layer as one of the organic thin-film layers, wherein said hole transport layer or electron transport layer contains the compound of the above-described formula (I) singly or as a mixture.
The use of the compound according to the present invention as a component material of an organic EL device makes it possible to provide light emission with higher brightness compared with the conventional device. Thus, the present invention brings about marked effects.